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to centrifugal force. Where screens of this type are employed, special devices must be used to hold the feed or move it in a direction contrariwise to the motion of the screen; for, as must be evident, since the grains cling to the screening surface and there would otherwise be no screening action whatever. One of the devices employed to this end is a stationary vane in contact or very nearly in contact with the screen, which holds the material fed and causes the screen to slide over it. Revolving vanes and brushes are also employed the direction of rotation being contrariwise to that of the screen.
The ordinary sizes of centrifugal reel range from 24 to 39 in. in diameter and from 5 to 8 ft. long. The 24-in. reel runs from 180 to 200 r.p.m. The reel shown in Fig. 18 runs from 100 to 130 r.p.m. and is made in diameters ranging from 24 to 30 in. The figure shows a vane revolving contrariwise to the motion of the screen.
Blinding of Screens and Methods of Prevention.—Blinding is of two kinds: (1) agglutinative, as with flour mill products, which is relieved by brushing or rubbing on yielding screens; and (2) of the sort where the particles are not de-
FIG. 19.—Grizzly sections.
formed but are jammed in the apertures. Square apertures are worse than round in this respect. In flour bolting machinery, loose discs of various shapes are used, being allowed to slide around freely on the bolting cloth. Chains also are used for the same purpose. For revolving screens, particularly those of polygonal shape which lend themselves to a ready adaptation, tappet devices are employed; but these are not very helpful and have the disadvantage of loosening bolts and other parts to the screen. For ores and rocks the occasional beating out of the revolving screen with a piece of belting mounted on a handle is probably about as efficacious a method of clearing the apertures as is practicable. At one of the Western ore mills relief from excessive blinding of a fine screen was obtained by mounting the trommel on a sufficiently light shaft, so that this member and the screen, by suitable means, could be kept in a state of rapid vibration.
For flat screens, rope beaters with a number of tappets have been proposed, one end of the rope being secured to the discharge end of the screen and the other fixed to the frame of the actuating mechanism. Traveling brushes and scrapers are also employed. (See Figs. 1 and 2 in the next section.) When the rope with its attached tappets is drawn taut by the advance of the screen, the tappets give the under side a sharp blow. Endless-belt screens may possibly get relief from blinding to some extent by the wedged grains being forced out on passing over rollers. Roller systems